October 24, 2012

Humans Evolved To Live Longer Because Of Grandmothers

Our grandmothers being there to care of us is something we all take for granted, and we have our ancestors to thank for that. New research indicates that human longevity is what it is because of grandmothers helping with childcare at an early stage in human history. Computer simulations on evolution have helped scientists prove that humans evolved longer life spans than apes because of their grandmotherly duties.

When it comes to chimpanzees, our closest primate relatives, females rarely live beyond their 30s, usually when their fertility usually ends. Although, based on the computer data, these female primates could evolve to extend their lifespan to those on par with human levels within 60,000 years if they took on a more grandmotherly role.

Previously, anthropologists have been divided as to whether humans´ long lives were due to the ℠grandmother hypothesis´ or the ℠hunting hypothesis.´ The computer simulations now show a stronger tie to the grandmother hypothesis.

“Grandmothering was the initial step toward making us who we are,” said Kristen Hawkes, a distinguished professor of anthropology at the University of Utah and senior author of the new study published in today´s issue of the British journal Proceedings of the Royal Society B.

In the computer experiments, simulated creatures, generally lived another 25 years after reaching adulthood, much like chimps. But after 24,000 to 60,000 years of grandmothering duties, the simulated creatures lived another 49 years, on par with human hunter-gatherers.

The grandmother hypothesis states that when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals. This theory also indicates that children becoming younger at weaning but older when they can first feed themselves and when they reach adulthood. The hypothesis also indicates that women who take on grandmotherly duties also end up with lifespans that go well beyond menopause.

Furthermore, by allowing their daughter to have more children, ancestral females who lived long enough to become grandmothers passed their longevity genes on to their descendants, who had longer lifespans as a result.

Another suggestion, based on the simulations, is that grandmothers may have even been responsible for increasing humans´ brain size by allowing mother to have larger families, increasing the pressure of natural selection. Bigger brains made early humans more capable of learning better hunting techniques and clever use of hunting weapons. The increased brain size in our ape-like ancestors was the major factor in humans developing lifespans longer than apes.

Hawkes conducted her study with the aid of mathematical biologist Peter Kim, a former University of Utah postdoctoral researcher now with University of Sydney, and also James Coxworth, a University of Utah doctoral student in anthropology.

Her grandmother hypothesis was first proposed in 1997, after observations in the 1980s when Hawkes and her then-research assistant James O´Connell lived with Hazda hunter-gatherers in Tanzania. They watched older women spend their days collecting tubers and other foods for their grandchildren, a unique trait in humans, as all other primates and mammals collect their own food after weaning.

But because of a lack of mathematical underpinning, Hawkes´ hypothesis has been heavily debated over the years. Now, the new study has sought to provide that foundation.

This new study was also a continuation on Hawkes´s past studies, especially a study from 1998 that took a similar, yet simpler approach, showing that grandmothering accounts for differences between humans and modern apes in life-history events such as age at weaning, age at adulthood and longevity.

However, another recent simulation suggested there were too few females living past their fertile life for grandmothering to affect lifespan in early humans.

Hawkes´ skepticism of those findings led her to undertake the new study, simulating evolution over time, and asking: “If you start with a life history like the one we see in great apes — and then you add grandmothering, what happens?”

Using the new simulation, Hawkes and her colleagues sought to measure the change in adult longevity--the average lifespan from the time adulthood begins. In the primate world, chimps that reach adulthood (age 13) live on average another 15 to 16 years. But for humans, those who live in developed nations who reach adulthood (around age 19) live an average of another 60 years.

To be conservative with their simulations, the team made the grandmother effect “weak;” they team assumed women couldn´t become grandmothers until age 45 or after age 75, that they couldn´t care for a child until age 2, could only care for one child and that it could be any child, not just the child of their daughters.

The simulation also took into account that any newborn had a 5 percent chance of a gene mutation that could either lead to a shorter or longer lifespan.

The simulation started with only 1 percent of women living to grandmother age and being able to care for grandchildren, but after 24,000 to 60,000 years in simulation, about 43 percent of adult women became grandmothers, a level similar to human hunter-gatherer populations seen today. The simulations also showed adult lifespan doubled from 25 to 49 years over the time span.

The difference in how fast the doubling occurred depends on different assumptions about how much a longer lifespan costs males. Longer life means more energy and metabolism is used to maintain bodies longer. In return, these males put less energy into competing with other males over females during young adulthood, something bigger brains allowed to happen.

However, the researchers wanted to know which came first: bigger brains or grandmothering?

The hunting hypothesis maintains that as resources dried up for our human ancestors in Africa, hunting became better than foraging for finding food, which led to natural selection for bigger brains. Under this scenario, women formed “pair bonds” with men who brought home meat.

The general consensus among anthropologists is that increasing brain size in our ape-like ancestors was the major factor in humans developing longer lifespans. But Hawkes´ grandmother hypothesis, backed up by the new computer simulation, ignored brain size, hunting and pair bonding, and showed even the weaker grandmother effect made simulated creatures evolve longer lifespans.

So Hawkes and her colleagues believe the shift to longer lifespans caused by grandmothering “is what underlies subsequent important changes in human evolution, including increasing brain size.”

“If you are a chimpanzee, gorilla or orangutan baby, your mom is thinking about nothing but you,” Hawkes said in a press release. “But if you are a human baby, your mom has other kids she is worrying about, and that means now there is selection on you — which was not on any other apes — to much more actively engage her: 'Mom! Pay attention to me!´”

“Grandmothering gave us the kind of upbringing that made us more dependent on each other socially and prone to engage each other's attention,” added Hawkes. This gave rise to “a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation.”

The fact of the matter, the study concludes, is that with only a little bit of grandmothering--and without assumptions about human brain size--it takes less than 60,000 years for animals with chimpanzee lifespans to evolve to have human lifespans.